This paper presents a filled function method to solve economic load dispatch (ELD) of the thermal unit. The proposed method can take care of ELD considering nonlinearity such as valve point loading. The effectiveness of the proposed algorithm has been verified on three different test systems with equality and inequality constraint. Simulation results show that the filled function method is less sensitive to initial values of parameters and is more effective than other previously developed algorithms. In addition, the proposed algorithm is highly promising for the large-scale system.